ABSTRACT

This poster shows our current understanding of the in-situ stress regime of the Western Canada Sedimentary Basin. Earth stresses are vector quantities characterized by direction and magnitude. Generally it may be assumed that the principal stress is vertical. Horizontal stress directions are indicated by borehole breakouts, hydraulic fracture orientations, overcoring measurements, special core analyses, and various geological indicators. Of these data, the most abundant in the basin are breakouts. Mean breakout azimuths from 175 wells have permitted the contemporary stress trajectories to be mapped for Paleozoic and Mesozoic rocks for the entire sedimentary section. An overall northeast - southwest direction is indicated for the larger horizontal principal stress with local variations around the Peace River and Sweetgrass arches.

Reliable stress magnitudes are more difficult to obtain. Vertical stress magnitudes may be equated with the weight of the overburden, thus their gradients are a function of rock density. Horizontal stress magnitudes appear to be sensitive to rock type and pore pressure. They generally increase with depth, although this increase is not necessarily linear at the formation scale. Quite often there is considerable variability in the measured stress magnitudes even over a short vertical interval. The most reliable horizontal stress magnitude measurements made in the basin are the smaller or minimum horizontal stress values obtained from microfracture tests (injected volumes typically less than 1 m³). Unfortunately, there are relatively few such records available since the practice of measuring in situ stress profiles in the petroleum industry is relatively new. There are, however, an abundance of less reliable minifracture data, which are prefracture stimulation tests pumped with viscous fluids at high rates and volumes usually in excess of 10 m³. Stress magnitude data from overcoring and strain relaxation techniques account for a small percentage of the available dataset. At the present time, it is difficult to discern how the horizontal stress magnitudes vary spatially across the basin.

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